In recent years, an Nd—Fe—B type alloy as an alloy for magnets is abruptly growing in production because of its superior properties, and being used for HD (hard disk), MRI (magnetic resonant imaging) or various motors. In general, Nd (denoted as R) with a part being replaced with another rare earth element such as Pr and Dy, or Fe (denoted as T) with a part being replaced with another transition element such as Co and Ni, is usually used and these including an Nd—Fe—B type alloy are generically called an R-T-B type alloy.
The R-T-B type alloy is an alloy comprising a crystal having, as the main phase, a ferromagnetic phase R2T14B contributing to the magnetization activity, where a non-magnetic, rare earth element-enriched and low-melting point R-rich phase is present at the grain boundary. This alloy is an active metal and therefore, is generally melted in vacuum or in an inert gas and then cast in a die.
The obtained alloy lump is usually ground into a powder material of about 3 μm (as measured by FSSS (Fisher sub-sieve sizer)), press-shaped in a magnetic field, sintered at a high temperature of about 1,000 to 1,100° C. in a sintering furnace and thereafter, if desired, subjected to heat treatment, machining and plating for corrosion prevention, whereby a magnet is completed.
The R-rich phase plays an important role in the following points.
1) The R-rich phase comes into a liquid phase at the sintering by virtue of its low melting point and therefore, contributes to densification of the magnet and in turn, enhancement of magnetization.
2) The R-rich phase eliminates the unevenness on the grain boundary to decrease reversed magnetic domains and enhance the coercive force.
3) The R-rich phase magnetically isolates the main phase and therefore, brings an enhanced coercive force.
As understood from these, bad dispersion of the R-rich phase adversely affects the properties of the magnet and therefore, uniform dispersion is important.
The R-rich phase distribution in a final magnet is greatly dependent on the structure of the raw material alloy lump. That is, when an alloy is cast in a die, crystal grains often grow due to the low cooling rate and therefore, the particles after grinding have a particle diameter by far smaller than the crystal grain diameter. Also, in the die casting, since R-rich phases are mostly aggregated at the grain boundary and not present within the particle, the particle containing only the main phase but not containing the R-rich phase and the particle containing only the R-rich phase are separately present and their uniform mixing becomes difficult.
As another problem in the die casting, γ-Fe is readily formed as the primary crystal due to the low cooling rate. The γ-Fe is transformed into α-Fe at about 910° C. or less and the transformed α-Fe incurs reduction in the grinding efficiency at the production of a magnet and if remains after sintering, deteriorates the magnetic properties. Therefore, in the case of an ingot cast from a die, the α-Fe must be eliminated by a homogenization treatment at a high temperature over a long period of time.
In order to solve these problems, a strip casting method (simply refereed to as an “SC method”) has been proposed as a casting method of realizing a cooling rate higher than that in the die casting method and this method is being used in actual processing.
In this casting method, a molten alloy is spread on a copper roll to cast a thin belt of about 0.3 mm, thereby effecting rapid cooling and solidification, as a result, the crystal structure is made fine and the alloy chip produced has a structure where R-rich phases are finely dispersed. The fine dispersion of the R-rich phase within the alloy chip leads to good dispersibility of the R-rich phase after grinding and sintering and in turn, the magnetic properties are successfully enhanced (see, Patent Document 1 (Japanese Unexamined Patent Application, Fists Publication No. H05-222488) and Patent Document2 (Japanese Unexamined Patent Application, Fists Publication. H05-295490)). However, also in this method, α-Fe is inevitably generated as the concentration of R component decreases and, for example, in the case of an Nd—Fe—B ternary alloy, generation of α-Fe is observed when Nd is 28 mass % or less.
This α-Fe conspicuously inhibits the grinding property in the step of producing a magnet.
The present inventors have made improvements of conventional centrifugal casting methods and invented a method of disposing a reciprocating box-type tundish with a plurality of nozzles on the inner side of a rotating mold, and depositing and solidifying a molten alloy on the inner surface of the rotating mold through the tundish (centrifugal casting, hereinafter simply referred to as a “CC method”), as well as an apparatus therefor (see, Patent Document 3 (Japanese Unexamined Patent Application, Fists Publication No. H08-13078) and Patent Document 4 (Japanese Unexamined Patent Application, Fists Publication No.8-332557)).
In the CC method, a molten alloy is sequentially poured on an already deposited and solidified alloy lump and since the additionally cast molten alloy solidifies while the mold makes one rotation, the solidification rate can be elevated. However, even in this CC method, when an alloy having a low R component concentration is intended to produce, α-Fe is inevitably produced due to the low cooling rate in the high-temperature region.
In order to avoid the production of α-Fe, the present inventors have invented a centrifugal casting method of sprinkling a molten alloy from a rotating tundish and depositing it on a rotating mold, so that the depositing rate of the molten alloy can be more decreased and thereby, the solidification and cooling rate in the CC method can be elevated (new centrifugal casting, hereinafter simply referred to as an “NCC method”, see Patent Document 5 (Japanese Unexamined Patent Application, Fists Publication No.2002-301554)). By this method, the generation of α-Fe is suppressed and as means for enhancing the magnetization properties of a magnet, a cast lump containing substantially no α-Fe on the low R component concentration side is obtained. Also, there has been proposed a method of depositing and solidifying a molten alloy on the inner surface of a rotating cylindrical mold with the inner surface being a convex and/or concave uneven face, so that the R-rich phase can be finely and uniformly distributed (see, Patent Document 6(Japanese Unexamined Patent Application, Fists Publication No.2003-77717)).
Furthermore, a depositing and solidifying method using a cylindrical mold has been proposed, where a film having a thermal conductivity smaller than that of the construction material of the mold is provided on the inner surface of the mold (see, Patent Document 7 (Japanese Unexamined Patent Application, Fists Publication No.2003-334643)).